A communication environment is currently being changed together with the development of a Broadband convergence Network (BcN) for allowing a service such as a wired and wireless complex network, a combination of a broadcasting network and a communication network, or IP convergence using an Internet protocol (IP) network. Such a tendency for change of the communication environment will be accelerated in future. Due to the change of the communication environment, terminals used in a variety of communication environments are being diversely changed and processing capabilities of the terminals are also being changed. Accordingly, in order to provide video signals optimized for various communication environments and various terminals, a variety of video signals provided to the terminals must be created. In order to provide the optimized video signals to the terminals, one video source must include a variety of combinations of variables such as the number of transmission frames per second, resolution and the number of bits per pixel, etc. This imposes a great burden on content providers.
In view of the above, the content providers encode an original video signal into high-bitrate compressed video data, decode the compressed video data into the original video signal when a request for the video is received from a terminal, and encode the original data into video data suitable to a video processing capability of the terminal before providing the video data to the terminal. However, in such transcoding (a combination of decoding and encoding), processes of encoding, decoding and encoding must be performed and thus a time delay occurs when providing the video signal to the terminal. Therefore, a device having complex hardware and algorithms is further required.
In order to solve such a problem, a scalable video coding (SVC) has been suggested. This method can represent video signals by encoding the video signals into a sequence of pictures with highest image quality and decoding only a part (a partial sequence of pictures intermittently selected from total sequence of frames) of the sequence of pictures. The sequence of pictures encoded using the SVC can reduce a video size using spatial scalability or reduce image quality using SNR scalability with a low bitrate. At this time, a sequence of pictures having a small screen size and/or a small number of frames per second is referred to as a base layer and a sequence of pictures having a large screen size and/or a large number of frames per second is referred to as an enhanced or enhancement layer.
Although it is possible to represent video signals in low image quality by receiving and processing a part of the sequence of pictures encoded in the scalable method as described above, the image quality is significantly degraded as the bitrate is lowered. One solution to this problem is to provide an auxiliary picture sequence having low bitrates, for example, a sequence of pictures that have a small screen size and/or a small number of frames per second. The auxiliary picture sequence is referred to as a base layer, and a main picture sequence is referred to as an enhanced (or enhancement) layer.
In this SVC, if a part of enhanced layer bitstream is lost when decoding a specific picture including the enhanced layer, which is encoded and transmitted, a decoder decodes the picture using the lost enhanced layer bitstream. Accordingly, the image qualities of the original video signals and the decoded video signals are different from each other. Specifically, when the picture having the above-mentioned problem is a reference picture necessary for decoding the other pictures as well as a key picture, the problem becomes more severe.